This invention relates to a multilevel quadrature amplitude demodulator for use as a counterpart of a multilevel quadrature amplitude modulator for modulating a main data signal into a multilevel quadrature amplitude modulated signal. Such a demodulator is particularly useful in an LSI (large scale integration circuit) modem for a digital communication system.
In the digital communication system, the multilevel quadrature amplitude modulator is used in effectively utilizing a radio frequency band as known in the art. The multilevel quadrature amplitude modulator modulates a pair of quadrature phase carrier signals by a main data signal into a multilevel quadrature amplitude modulated signal. A transmitter is supplied with the multilevel quadrature amplitude modulated signal and frequency converts the multilevel quadrature amplitude modulated signal into a radio frequency signal of a microwave band to transmit the radio frequency signal through a transmission path.
A receiver receives the radio frequency signal through the transmission path and frequency converts the radio frequency signal into the multilevel quadrature amplitude modulated signal of an intermediate frequency band.
The multilevel quadrature amplitude demodulator is supplied with the quadrature amplitude modulated signal as a demodulator input signal having an input signal phase. The multilevel quadrature amplitude demodulator comprises a processing unit and a phase control circuit. The processing unit processes the demodulator input signal for producing inphase and quadrature processed signals. The inphase and the quadrature processed signals have inphase and quadrature signal phases, respectively. The processing unit comprises a quadrature phase detector. The quadrature phase detector carries out phase detection of the demodulator input signal by using first and second local carrier signals each of which has a constant frequency and produces inphase and quadrature baseband signals. The second local carrier signal has a quadrature phase difference relative to the first local carrier signal. Each of the inphase and the quadrature baseband signals is supplied to the phase control circuit through a low-pass filter, an analog-to-digital converter, and a digital filter as will later be described more in detail.
The phase control circuit controls the inphase and the quadrature signal phases so that each of the inphase and the quadrature signal phases coincides with the input signal phase. The phase control circuit reproduces inphase and quadrature demodulated signals at a certain code error rate.
If a quadrature phase deviation, namely, a phase deviation in orthogonality, occurs between the first and the second local carrier signals, the code error rate is seriously deteriorated. The quadrature phase deviation is caused by performance of the quadrature phase detector. This is because the quadrature phase detector is influenced by aged deterioration and a variation in ambient temperature. The quadrature phase deviation should be kept as little as possible. It is therefore desirable to design exactly for the quadrature phase detector so as to reduce influence of the aged deterioration and the variation in ambient temperature. Such an exact design is, however, objectionable because the quadrature phase detector becomes expensive.